Method for forming improved coatings on metal



ted States Patent 3,397,091 METHOD FOR FORMING IMPROVED COATINGS 0N METAL William S. Russell, Warren, Mich., assignor to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed Mar. 9, 1965, Ser. No. 438,396 15 Claims. (Cl. 148-62) ABSTRACT OF THE DISCLOSURE A method for forming a protective coating on zinccontaining surfaces wherein a clean, zinciferous surface is coated with an aqueous acidic solution which contains (hexavalent chromium ions, fluoride ions, and from 0.01 to 0.1% by weight of the aqueous solution of arsenic ions. After the zinc metal surface has been contacted with the solution for a period suflicient to form the desired coating thereon, the thus-coated surface is then rinsed with an aqueous solution containing hexavalent chromium ions.

This invention relates to an improved process for coating metal surfaces and more particularly it relates to an improved process for chemically coating zinc-containing surfaces to provide a corrosion resistant and paint bonding coating on such surfaces.

Many compositions and processes have been heretofore proposed for the treatment of zinc-containing surfaces, i.e., surfaces of zinc and alloys which are predominantly zinc, to produce a chemical coating thereon. In many of these, and particularly those which have been the most successful commercially, hexavalent chromium has been employed as the principal coating material. Fluoride ions, as Well as various other anions or cations to provide increased coating weight and coating efliciency, have also frequently been used in compositions of this type, with the hexavalent chromium. In all these processes, it has been attempted to provide a coating solution which is relatively simple to prepare and which is easily maintained in effective coating condition during use. Additionally, it has frequently been found to be desirable that the coating methods used are not only effective in coating zinc-containing surfaces but are also useful in treating various types of aluminum and aluminum alloys. In this way, the same method and composition can be effectively used on so-called mixed production, without the necessity of interrupting the coating process to change solutions each time it is desired to coat a different type of metal. For the most part, however, the processes of the prior art have not been successful in achieving all of these objectives.

It is, therefore, an object of the present invention to provide an improved process for coating zinc-containing surfaces which process is also effective in coating other metals, such as aluminum.

Another object of the present invention is to provide an improved method for coating zinc-containing surfaces wherein the coating solution used is easily prepared and maintained in acceptable coating condition.

These and other objects of the invention will become apparent to those skilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention includes a method for forming a coating on zinc-containing surfaces which comprises contacting a clean zinc-containing surface with a coating composition which comprises an aqueous acidic solution which contains hexavalent chromium ions, fluoride ions, and from 0.01 percent to 0.1 percent of arsenic ions, the amount of said ions being calculated as the metal, maintaining the solution in contact with the surface for a period of time sufficient to form the desired coating, removing the thus-coated surface from contact with the solution and, thereafter, contacting the thus-coated surface with a rinsing composition which comprises an aqueous solution containing hexavalent chromium ions.

The aqueous acidic coating solutions used in the method of the present invention contain hexavalent chromium ions in an amount suflicient to provide the desired chromium coating on the zinc surfaces which are treated therewith. Desirably, these solutions contain hexavalent chromium ions, calculated as CrO in an amount within the range of about 0.05 percent to about 1 percent by weight of the solution. The hexavalent chromium ions may be added to the solution in many suitable forms, such as chromic acid, or one or more of the water soluble or water dispersible salts of chromic acid. Exemplary of the salts which may be used are the alkali metal or ammonium salts, such as the alkali metal or ammonium chromates and d'ichromates, as well as mixtures thereof, both with each other and/ or with chromic acid.

The fluoride ion is present in the composition in an amount suflicient to cause attack of the zinc-containing surface to be coated and to effect formation of the resulting coating on this surface. Desirably, the fluoride ion is present in an amount within the range of about 0.05 percent to about 2.7 percent by weight of the solution. As with the hexavalent chromium ions, the fluoride ions may be added to the composition in many convenient forms, including various fluorine-containing compounds which are capable of ionizing iodine aqueous acidic solutions to provide fluoride ions. Exemplary of such fluorine-containing compounds which may be used are hydrofluoric acid, fluosilicic acid, fluoboric acid, as well as the various water soluble or water dispersible salts thereof, such as the alkali metal and ammonium salts.

As has been indicated hereinabove, there is also included in the coating composition used in the method of the present invention from 0.01 percent to 0.1 percent by weight of the solution of arsenic ions. These ions may be added to the solution in the form of various compounds which are ionizable in the coating solution, such as arsenic acid, as well as the various water soluble or water dispersible salts of arsenic acid, which will provide the desired arsenic ion when oxidized in the solution by the chromic acid or salts thereof. Exemplary of such salts which may be used are the alkali metal or ammonium salts of arsenic acid.

Additionally, where the compositions used in the present method contain hydrogen fluoride as the source of the fluoride ions, it may also be desirable to include in the composition a quantity of a buffering acid, such as boric acid or silicic acid, to act as a buffer for the fluoride ions. Where these acids are included in the coating solution, they are typically present in amounts within the range of about 0.1 percent by Weight of the solution up to the maximum solubility of the acid in the solution. Desirably, however, these acids are present in the composition in amounts within the range of about 0.1 percent to about 0.2 percent by weight of the coating solution.

In addition to the above components, in many instances it has also been found desirable to include in the subject composition aluminum, preferably as an aluminum fluoride complex ion. Desirably, the aluminum is present in an amount within the range of about 0.01 percent to about 1.0 percent by weight of the coating solution, although amounts up to the maximum solubility of the aluminum compound added may be used. The desired aluminum fluoride complex ion is expressed as Al(F) because, when used, it is present in the operating solution as an equilibrium of Al(F) ions which may contain from 1 to 6 fluoride atoms per aluminum atom. In the solutions of the present invention, it has been found that this equilibrium averages out to be approximately equivalent to the AlF ion. Accordingly, as used in the specification and claims, the expression Al(F) is intended to represent any aluminum fluoride ion and the quantities thereof refer to an amount of such ion equivalent to the AlF ion. This aluminum fluoride complex ion, when used, may be incorporated in the present composition as such or it may be formed in the composition as a complex from free aluminum and fluoride ions. In the latter instance, the fluoride may be present as hydrofluoric acid, fluoboric acid, fluosilicic acid, or the like. Where the aluminum fluoride complex ion is added as such, it may be prepared by dissolving aluminum oxide (A1 in water and hydnofluoric acid in appropriate proportions to obtain the requisite parts of AlF for the composition.

Particularly preferred compositions for use in the pres ent method are those falling within the following formulation:

Concentration in Solution component: percent by weight Hexavalent chromium ions (calculated as CrO 0.1 to 0.5 Fluoride ions 0.05 to 1.6 Arsenic ions 0.03 to 0.1 Buffering acid Nil to 0.2 Al(F) complex Nil to 1.9

In carrying out the process of the present invention, the aqueous coating solutions as described hereinabove, are applied to a cleaned zinciferous surface. By a cleaned zinciferous surface is meant a zinc-containing surface which is substantially free of foreign matter, such as oil, grease, dirt, and the like. The cleaning of the zinciferous surface may be effected by contacting the surface with various alkaline cleaning solutions, such as aqueous solutions containing alkali metal hydroxides, alkali metal carbonates, alkali metal phosphates, alkali metal silicates, and the like. Exemplary of the alkali metal phosphates which may be used in the alkaline cleaning solution are the trialkali metal phosphates, the tetraalkali metal pyrophosphates, the alkali metal tripolyphosphates, and the like. It will be appreciated that as used in the specification and claims, the term alkali metal is intended to refer to lithium, sodium, potassium, cesium and rubidium. The preferred alkali metal is sodium, however, and primary reference will be made hereinafter to sodium compounds. This is not, however, to be taken as a limitation of the alkali metal compounds which may be used as other alkali metal compounds, and particularly potassium compounds, have been found to be suitable for use in the process of the present invention.

It has been found that in addition to cleaning the zinciferous surfaces, in some instances these alkaline solutions also have an activating effect on the surface which results in obtaining an improved paint bonding coating upon the subsequent application of the coating composition. Accordingly, although various alkaline materials may be used in formulating the aqueous solution, such as the hydroxides and/or carbonates of sodium and potassium, trisodi'um, or tripotassium phosphate, and the like, the preferred alkaline solutions in terms of the activation of the zinciferous surface, are those solutions which contain an alkali metal metasilicate, such as sodium metasilicate. Exemplary of typical alkaline solutions which may be used are those containing the alkaline material in an amount within the range of about 4 grams to about 28 grams per liter and having a pH within the range of about 9.5 to about 13.5. A particularly desirable composition for cleaning and activating these zinciferous surfaces is one having the following formulation:

Percent by weight Sodium metasilicate 50 Trisodium phosphate 50 This dry composition is formulated into an aqueous cleaning solution by admixing it with water in amounts within the range set forth above.

The aqueous alkaline solution may be applied to the zinciferous surface to be treated using various techniques, such as immersion, flooding, spraying, and the like, with spraying techniques generally being preferred. Desirably, the alkaline solution at the time of application to the zinciferous surface is at an elevated temperature, with temperatures within the range of about 45 degrees centigrade to about 90 degrees centigrade being typical, and temperatures within the range of about 65 degrees centigrade to about degrees centigrade being preferred. The hot alkaline solution is maintained in contact with the surface to be treated for a period of time suflicient to effect cleaning and the desired activation of the zinciferous surface. Generally, contact times up to about 2 minutes are typical, with contact times of about 10 seconds to about 1 minute being preferred. After cleaning, the surface may be rinsed with water, preferably using a hot water spray with the water at a temperature within the range of about 50 degrees Centigrade to about 80 degrees centigrade. After the cleaning. and rinsing of the zinciferous surface has been accomplished, the coating solution, as described hereinabove, may be applied thereto using various coating techniques, such as dipping, brush ing, spraying, flooding, roller coating, or the like. Additionally, the coating solutions may be applied to the zinccontaining surfaces by atomizing the solution on the surface in a heated condition, in accordance with the procedure set forth in a co-pending US. patent application Serial Number 728,095, filed April 14, 1958. Generally stated, this atomization application technique includes the step of preliminarily heating the zinc or zinc alloy surface to be coated to a temperature above about 65 degrees centigrade and atomizing on the heated surface a quantity of the coating solution sufficient to form the desired coating but insuflicient to cause the droplets of atomized coating solution to coalesce or puddle on the surface. The coating on the zinc surface results from the substantially instantaneous flashing or volatilization of the liquid from the solution, so that each individual atomized particle droplet remains substantially in the locus of its original contact with the surface treated.

Although the coating process of the present invention may be carried out effectively and with good efliciency over a wide range of solution temperatures and solution acidities, it has been found that the rate of coating may be improved and the coating efficiency increased by the concurrent selection and control of the degree of the acidity of the solution and the temperature of application. With regard to the temperature, it has been found that as the temperature of the operating solution is increased from room temperature, i.e., about 20 degrees centigrade up to about 50 degrees centigrade, the coating rate rapidly increases and in some instances it is possible to obtain an increase in the coating rate of from 2 to 5 times that obtained at room temperatures. At solution temperatures within the range of about 50 degrees centigrade to about 70 degrees centigrade, the coating rate has been found to increase more slowly and, in many instances, has been found to be substantially uniform throughout this temperature range. It is, therefore, preferred to utilize the solutions in the present coating method at solution temperatures within the range of about 50 degrees centigrade to about 70 degrees centigrade. Higher temperatures than 70 degrees centigrade may be employed, for example, temperatures of 80 degrees centigrade or even up to the solution boiling point, but no particular advantages in terms of increased coating rates are obtained by operating at such higher temperatures.

With regard to the pH of the operating solutions of the present invention, it has been found that these, as with the temperatures, effect the coating rate and coating efficiency of the solution being applied to the zinccontainin-g surface. Accordingly, it is desirable that the coating solution have a pH within the range of about 1.3 to about 3.2, and preferably in the range of about 1.7 to about 2.2. This pH range refers to measurements taken by using an electrical pH meter employing a glass electrode and a calomel electrode, by immersing the electrodes in fresh portions of the operating solution and observing the indicated values.

In addition to the pH of the operating solution, it is also desirable that this solution have a concentration within the range of about 2 to about 30 points and that once the concentration is established within this range, it is maintained within about 10.5 point of the established value. The concentration of the operating solution in points is determined by the following procedure:

To a milliliter sample of the operating solution there is added 25 milliliters of 50 percent sulfuric acid and 2 drops of orthophenanthroline ferrous complex (ferroin) indicator. This solution is then titrated with 0.1 N ferrous sulfate in dilute sulfuric acid until the solution changes through blue to a reddish brown color. The concentration points of the operating solution are the number of milliliters of the 0.1 N titrating solution used. It is to be appreciated, that although the operating solution of the present invention is desirably used at a concentration Within the range of about 4 to 30 points, operation of the solution at both higher and lower point concentrations is not only possible, but in some instances, is preferred.

Following the application of the coating solution to the zinc-containing surfaces to be treated, the thus-coated surfaces are then desirably rinsed with water. Either spray or immersion techniques for the water rinse may be used, with rinsing times of about 3 to 5 seconds duration being typical.

Thereafter, the zinc-containing surface is contacted with a rinsing composition, which composition comprises an aqueous solution containing hexavalent chromium ions. This rinsing composition is a water solution containing a source of hexavalent chromium, calculated as CrO in an amount within the range of about 0.03 percent to about 0.8 percent by weight of this solution, and preferably in an amount within the range of about 0.07 percent to about 0.3 percent by weight of the solution. Various water-soluble or water dispersible sources of hexavalent chromium may be used in formulating the solution, provided the anions and cations introduced with the hexavalent chromium do not have a detrimental effect on either the solution itself or the coated zinc surfaces which are treated. Exemplary of hexavalent chromium materials which may be used are chromic acid, the alkali metal and ammonium chromates, the alkali metal and ammonium dichromates, the heavy metal dichromates, such as the dichromates of Zn, Ca, Cr, 1%, Mg and Al, and the like. This rinsing composition may be applied to the coated zinc surface using various means, including roller, immersion, flooding, and spraying techniques, and the like, with spraying techniques being preferred. Additionally, if desired, after the application of the hexavalent chromium solution to the surface, the excess of this solution may be removed from the surface, preferably by Wiping or squeegeeing. Generally, it is preferred that the aqueous hexavalent chromium-containing solution is maintained at an elevated temperature while it is in contact with the zinc surface to be treated. Temperatures wthin the range of abou 35 degrees centigrade to about 60 degrees centigrade and contact times of up to about 60 seconds are typical. After contacting the coated zinc-containing surface with the rinsing composition, and removing excess liquid from the surface by wiping or squeegeeing, the surface may be dried, temperatures within the range of about degrees centigrade to degrees centigrade and times up to about 5 minutes being typical of those used in such a drying operation.

The coatings thus produced on the zinc surfaces are slightly colored and vary in appearance from iridescent to light gold to yellow to brown. The color changes in the coating produced may be used as a general guide to the coating weights obtained, the darker colors being produced with higher coating weights and the lighter colors resulting from lower coating weights.

In formulating the operating coating solution for use in the present invention, a make-up composition containing the components desired in the operating solution, is admixed with water in amounts suitable to provide concentrations of the components within the ranges as has been set forth hereinabove. Normally, in addition to the above indicated components, the make-up composition may also contain an inorganic mineral acid, such as nitric acid, sulfuric acid, hydrochloric acid, or the like, in order to provide the desired acidity or pH. Suitable make-up compositions are those falling within the following formulations:

It will be appreciated that this is a single package makeup composition as compared to the prior art composition wherein separate packaging of some of the make-up components was often necessary.

In the operation of the process of the present invention, the components of the operating coating solution are depleted. Accordingly, in order to maintain these components in the operating solution within the preferred ranges which have been heretofore given, it is desirable in order to obtain a continuous operation, to periodically replenish the operating solution. One advantage of the present composition is that, as in formulating the operating solution, this replenishing may be effected using a single package replenishing material, as opposed to many of the prior art compositions wherein separate addition of the components during replenishing is necessary. As with the make-up composition, in addition to the hexavalent chromium fluoride and arsenic components, it may also be desirable to include in the replenishing material an inorganic mineral acid, such as nitric acid, sulfuric acid, hydrochloric acid, or the like, so as to maintain the acidity or pH of the operating solution within the desired ranges as have been indicated hereinabove. Additionally, where the fluoride components are added as hydrogen fluoride rather than fluosilicic acid or fluoboric acid, it may also be desirable to include in the replenishing material a quantity of boric acid or silicic acid to act as a buffer for the fluoride ions.

A single package replenishing material suitable for use in maintaining the operating solution of the present invention in optimum coating forming conditions may contain the following components in the amounts indicated:

Components: Parts by weight Hexavalent chromium (calculated as CrO 15 to 20 HF 1 to 5 Inorganic mineral acid 2.0 to 20 Sodium arsenate 2 to 10 Boric acid to 2 AlF complex 0 to A preferred replenishing material having particular utility for use with a continuous strip line operation, is one containing the following components in the amounts indicated:

A preferred replenishing material, particularly suitable for production operations in which the parts to be coated are moved through a spray installation on a monorail conveyor, is one having the following components in the amounts indicated:

Components: Parts by weight CrO 15 to 17 HF 2 to 4 HNO 8 to 16 Sodium arsenate 4 to Boric acid 0 to 2 AlF complex 0 .to 5

In some instances, it has been found to be advantageous to operate the solutions of the present invention in conjunction with cation exchange units. In this manner, the amount of mineral acid in the composition may be reduced and the consumption of fluoride ions may also be decreased.

In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given. It is to be understood that these examples are presented for illustrative purposes only and are not intended to be taken as a limitation of the present invention. In these examples, unless otherwise indicated, temperatures are in degrees centigrade and amounts are in percent by weight.

In the following examples, 1-14, panels of hot dip galvanized steel were cleaned by immersion for to seconds in an aqueous, alkaline cleaning solution containing about 15 grams per liter of a -50 mixture of sodium metasilicate and trisodium phosphate. The cleaning solution was at a temperature of about degrees centigrade. Thereafter, the cleaned panels were rinsed in a hot water spray at about 65 degrees centigrade. The panels were then sprayed with the coating solutions indicated hereinbelow. These solutions were formulated by mixing the components in the quantities indicated with suflicient water to form about six liters of solution. The CrO was added as chromic acid, the fluoride was added as fluoboric acid, except in Examples 6 and 9 where it was added as hydrofluoric acid, the N0 was added as nitric acid and the As O was added as sodium arsenate. The coating solutions were at a concentration level of about 11 points, a temperature of about 60 degrees centigrade and the spraying time was about 15 seconds. Thereafter, the coated panels were rinsed in a second hot water spray and were then sprayed with an aqueous rinsing solution containing about 0.1 percent by weight of CrO The rinse solution was at a temperature of about 50 degrees Centigrade and the contact time was about 15 seconds. The excess liquid was removed from the panels with a squeegee and the panels were dried for 2 minutes at 110 degrees centigrade. Several panels from each example were then painted with a white vinyl paint and subjected to the standard 5 percent salt spray, percent humidity, knife blade adhesion and physical deformation tests. In each instance, high ratings were obtained, indicating the formation of an excellent protective and paint bonding coating on the panels under the paint. Additionally, all of the panels were weighed before cleaning and after drying. In all cases, the weight loss in these panels was less than about 15 milligrams per square foot as compared to control panels treated with the same coating solution, but not containing the activator materials, Which control panels all had at least a 50 percent greater weight loss. With regard to the weight loss, this is a measure of the effectiveness of the coating process, the lower the weight loss, the greater the amount of coating applied and the more effective the coating process. The difference in weight loss between the control panels and those treated with the activated compositions is not necessarily the coating weight applied. The actual coating weight applied cannot be easily determined because there is no satisfactory way of stripping the coating from the coated zinc panels.

Components in percent by weight of coating 5 ution Examples pH CIO; F AS205 HNOa H3130;

U. 05 O. 05 0. O9 1. 6

EXAMPLE 15 The procedure of the preceding examples was repeated with the exception that a coating solution, aged for about 72 hours, was used. This solution, at a concentration of about 11 points, had a pH of 1.58. The solution, in an amount of six liters, was formulated as in the previous examples and contained the following components in the amounts indicated:

Percent by weight of Components: coating solution CrO 0.38 Zn+ 0.39 Cr 0.29 Al+ 0.19 No 1.76

Sodium arsenate Losses in milligrams additions (grams):

per square foot EXAMPLE 1-6 A coating solution was formulated as in the preceding examples, which solution contained the following components in the amounts indicated:

Percent by weight of Components: coating solution CrO Zn+ 1.35 Cr+ 0.44 Al+ 0.07 N 2.85 F 0.77 Sodium arsenate 0.10

The pH of this solution was 2.0. Hot dip galvanized stock was coated with this solution as in the previous examples. The coating obtained was a very good paint base and when painted provided excellent corrosion resistance. During the time the stock was coated with the above solution, the coating solution was periodically replenished, as required, to maintain the desired amounts of the various components, with a replenishing material having the following composition:

Components: Percent by weight CrO 4.80 HF 0.40 HNO 2.10 Sodium arsenate 0.50 'AlF complex 1.20 Water Balance While there have been described various embodiments of the invention, the compositions and methods described are not intended to be understood as limiting the scope of the invention as it is realized that changes therewithin are possible, and it is further intended that each element recited in any of the following claims is intended to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principles may be utilized.

What is claimed is:

1. A method of forming a protective coating on zinccontaining surfaces which comprises contacting a clean, Zinciferous surface with a coating composition which consists essentially of an aqueous acidic solution which contains an effective coating amount of hexavalent chromium ions, fluoride ions in an amount effective to attack the surface being treated, and from 0.01 percent to 0.1 percent by weight of the coating composition of arsenic ions, the amount of said arsenic ions being calculated as the metal, maintaining said solution in contact with the surface for a period of time sufficient to form the desired coating thereon, removing the thus-coated surface from contact with the solution and, thereafter, contacting the thus-coated surface with a rinsing composition which comprises an aqueous solution containing hexavalent chromium ions.

'2. The method as claimed in claim 1 wherein the coating composition contains 0.05 percent to 1 percent by Weight of hexavalent chromium ions, calculated as CIO;;, and 0.05 percent to 2.7 percent fluoride ions.

3. The method as claimed in claim 2 wherein the coating composition also contains 0.1 per-cent to 0.2 percent 'by weight of a buffering acid, effective in buffering the fluoride ions in the composition.

4. The method as claimed in claim 3 wherein the buffering acid is boric acid.

'5. The method as claimed in claim 3 wherein the coating solution also contains aluminum in an amount within the range of about 0.01 percent to about 1.0 percent by weight.

6. The method as claimed in claim 5 wherein the amounts of the components of the coating solution are maintained within the indicated ranges by periodically replenishing the solution with an aqueous composition comprising the following components in the amounts indicated:

7. A method for treating a zinc-containing surface which comprises contacting a zinciferous surface with an aqueous alkaline solution, maintaining the alkaline solution in contact with the surface for a period of time sufficient to effect cleaning of the surface, contacting the thuscleaned zinciferous surface with a coating composition which consists essentially of an aqueous acidic solution which contains an effective coating amount of hexavalent chromium ions, fluoride ions in an amount effective to attack the surface being treated, and from 0.01 percent to 0.1 percent by weight of the coating composition of arsenic ions, the amount of said arsenic ions being calculated as the metal, maintaining said coating solution in contact with the surface for a period of time sufficient to form the desired coating thereon, removing the thuscoated surface from contact with the solution and, thereafter, contacting the thus-coated surface with a rinsing composition which comprises an aqueous solution containing hexavalent chromium ions.

8. The method as claimed in claim 7 wherein the aqueous acidic coating solution contains from about 0.05 percent to about 1 percent by weight of hexavalent chromium ions, calculated as CrO and from about 0.05 percent to about 2.7 percent by weight of fluoride ions.

9. The method as claimed in claim 8 wherein the aqueous acidic coating solution also contains from about 0.1 percent to about 0.2 percent by weight of a buffering acid, effective in buffering the fluoride ions in the composition.

10. The method as claimed in claim 9 wherein the buffering acid is boric acid.

1 1. The method as claimed in claim 7 wherein the aqueous alkaline solution with which the zinciferous surface is first contacted is an aqueous solution of an alkali metal metasilicate.

12. The method as claimed in claim 9 wherein the aqueous alkaline solution with which the zinciferous surface is first contacted is an aqueous solution of an alkali metal metasilicate.

13. The method as claimed in claim 9 wherein the coating solution also contains aluminum in an amount within the range of about 0.01 percent to about 1.0 percent by weight.

14. The method as claimed in claim 13 wherein the amounts of the components of the coating solution are maintained within the indicated ranges by periodically replenishing the solution with an aqueous composition comprising the following components in the amounts indi- 15. A zinc surface having a coating thereon produced cated: in accordance with the method as claimed in claim 1.

Components: Parts by Weight References Cited Hexavalent chromium (calculated as 5 UNITED STATES PATENTS 9 i 3 2,494,909 1/1950 Spruance et al 1486.2

Inorganic mineral acid 2 to 20 FOREIGN PATENTS Sodium arsenate 2 to 10 846,363 8/ 1960 Great Britain.

Boric acid 0 to 2 10 AlF complex 0 to 5 RALPH S. KENDALL, Primal/y Examiner. 

